http://cdn.agilitycms.com/nhlpacom/PDF/NHL_NHLPA_2013_CBA.pdf
He has to be offered back to his junior club, but if they don't want him back they can loan him ... to anywhere including the AHL.
Age is not an issue, it is only the NHL-CHL agreement that he be offered back to his junior club.
Considering he is an import he occupies a position that is limited, and if they have selected someone they are higher on they might release him.
It really is up to his junior club whether they want to keep him. They could of course trade his rights if they wanted.
Odds are he's going back to junior, but the AHL would not restrict the Marlies from using him.
This is a contentious rule anyway that probably straddles the law as far as employment rules in Canada.